This invention relates to a electromechanical bypass for use in a hydrostatic transmission ("HST"). Hydrostatic transmissions are well known in the art, and are more fully described in, e.g., U.S. Pat. No. 5,314,387, which is incorporated by reference herein. This invention can also be adapted for use in an integrated hydrostatic transmission ("IHT") incorporating gearing and axles within a single housing.
In general, an HST has a hydraulic pump and a hydraulic motor mounted in a housing. The pump and motor are hydraulically linked through a generally closed circuit, and both consist of a rotatable body with pistons mounted therein. Hydraulic fluid such as oil is maintained in the closed circuit, and the HST generally has a sump or reservoir from which the closed circuit can draw oil from or dump oil to. This sump may be formed by the housing itself.
The pump is usually driven by an external motive source such as pulleys or belts connected to an internal combustion engine. The pump pistons engage a moveable swash plate and, as the pump is rotated by an input source driven by the external engine, the pistons engage the swash plate. Other HST designs may use a radial piston or ball piston pump and motor design, but the general operation is similar. Movement of the pump pistons creates movement of the hydraulic fluid from the pump to the motor, causing rotation thereof. The motor pistons are engaged against a fixed plate, and rotation of the motor drives an output shaft engaged thereto. This output shaft may be linked to mechanical gearing and axles to drive a vehicle, which may be internal to the HST housing, as in an IHT, or external.
The pump/motor system is fully reversible in a standard HST. As the swash plate against which the pump pistons move is moved, the rotational direction of the motor can be changed. In addition, there is a "neutral" position where rotation of the pump does not create any movement of the hydraulic fluid. However, in most designs this neutral band is very narrow, as it is dictated by the mechanical design of the unit and the user's ability to mechanically locate the neutral area through use of a shift lever or foot pedal system.
The HST closed circuit has two sides, namely a high pressure side in which oil is being pumped from the pump to the motor, and a low pressure, or vacuum, side, in which oil is being returned from the motor to the pump. When the pump direction is reversed, the two sides reverse, with the high pressure side becoming the vacuum side and vice versa. This circuit can be formed as porting formed within the HST housing, or internal to a center section on which the pump and motor are rotatably mounted, or in other ways known in the art. Check valves are often used to draw hydraulic fluid into the low pressure side to make up for fluid lost due to leakage, for example. Such check valves may be located directly in the porting or maybe located in a center section and connected to the closed circuit.
There is a need to have a means to open, or bypass, this closed circuit in certain circumstances. For example, when the vehicle is stopped, the oil in the closed circuit provides hydraulic braking, making it impossible to manually move the vehicle. Mechanical bypass designs are known in the art and are described in, for example, U.S. Pat. No. 5,010,733. Such designs generally achieve bypass by opening the closed hydraulic circuit to the sump by, e.g., opening check valves in the circuit, or by opening a shunt between the high pressure and low pressure sides of the circuit.
However, such prior art designs have drawbacks. For example, in addition to those identified above, a completely open hydraulic circuit can lead to uncontrolled free-wheeling of the vehicle and create significant safety risks. In addition, mechanical bypass mechanisms require various linkages from the HST and IHT to the vehicle, and it can be difficult for the manufacturer of the HST or IHT to accommodate more than one style of actuation of the mechanical bypass.